Staphylococcus aureus (S. aureus) is a leading cause of human infections worldwide, resulting in a diverse spectrum of disease severity from mild to life-threatening. Recently, there has been an increase in the incidence of community-associated methicillin-resistant S. aureus (CA-MRSA) infections in otherwise healthy individuals, the basis of which is largely unknown. Human polymorphonuclear leukocytes (PMNs or neutrophils) are the first line of defense against bacterial infections. The ability of S. aureus to circumvent destruction by innate immunity includes survival after PMN phagocytosis. The ability of S. aureus to survive following PMN phagocytosis is dependent on the pathogen's ability to sense and survive the hostile PMN environment. However, specific mechanisms used by S. aureus to evade PMN killing are incompletely defined. We hypothesize that the SaeR/S two-component regulatory system directly regulates S. aureus factors that impair an effective neutrophil response resulting in pathogen survival. This hypothesis is based on the following published and preliminary observations: 1) saeR and saeS transcripts were up-regulated following PMN phagocytosis; 2) deletion of saeR/S significantly decreased survival of S. aureus following human PMN phagocytosis; and 3) transcriptional assays identified extracellular virulence genes down- regulated in saeR/S and direct binding of recombinant SaeR to a consensus sequence within several of these virulence gene promoters. These data correlate well with in vivo studies that demonstrate SaeR/S regulates factors vital for S. aureus - induced morbidity following skin infections and mortality following invasive infections. To determine the molecular mechanisms behind the saeR/S-dependent phenotype we will first define the kinetics of activation of SaeR/S-regulated genes in response to PMN phagocytosis. Next, we will determine the contribution of individual SaeR/S-target genes to S. aureus survival and PMN lysis. Finally, we will assess expression of saeR/S-target genes in vivo and identify the contribution of individual genes regulated by saeR/S to S. aureus pathogenesis. Completion of this research will identify the specific effectors regulated by saeR/S responsible for survival following PMN phagocytosis and will define their contribution to PMN lysis and S. aureus pathogenesis in vivo. These findings will improve our understanding of the initial host-pathogen interactions that lead to S. aureus infection. This knowledge is predicted to have direct application for novel prevention and treatment strategies for staphylococcal infections.